Our objective is to identify and functionally characterize the genes and genetic pathways underpinning metazoan regeneration. We propose to use the planarian Schmidtea mediterranea as a model system to address this problem because it is among the simplest cephalized bilaterians with complex organ systems that display extensive regenerative capacities: a decapitated animal will regenerate and functionally integrate a new head to the pre-existing tissues in under a week. In addition, such remarkable developmental plasticity is driven by an abundant and experimentally accessible population of totipotent stem cells known as neoblasts. The advances made during the last period of funding to interrogate gene function and measure regenerative responses, combined with the sequencing and recent annotation of the S. mediterranea genome create an unprecedented opportunity to frame the problem of animal regeneration in molecular and functional genomic terms. We propose to take full advantage of the throughput capacity and biological plasticity afforded by planarians to establish a detailed molecular landscape of regeneration. We propose to: 1) carry out genome- wide, high temporal resolution analyses of regeneration using high density DNA arrays; 2) test the functions of key, evolutionarily conserved embryonic signaling pathways in the adult contexts of tissue homeostasis and regeneration to provide a functional basis for interpreting the microarray experiments; and 3) to carry out screens of organ-specific regeneration to determine the regulatory extent and degree of molecular specialization that exists in general regenerative events. Combined, these studies should provide us with the most comprehensive mechanistic study of regeneration performed to date, and should serve as a platform to formally test the evolutionary divergence or convergence of regeneration among animals, a central unresolved aspect hindering the implementation of rational regenerative therapeutics in poor regenerators such as mammals.